The name of the spatial index is derived from the acronym (I)n(D)e(X) (P)olygons at the (7500) foot tile level, or idxp7500.
Cell label is a row-id cocantenated with a column-id generating a four-character identifier that uniquely identifies every cell. The row portion of the identifier is a two-character alpha code of the format aa, ab, ac.... ba, bb, etc. and the column portion is a two-digit integer value such as 01, 02, 03.... 11,12,13, etc. A composite cell identifier would be then, for example, aa01, aa02, .....ba11,ba12, etc.
Not all image and raster data is stored at the tiling level represented by this index. Data is stored at this level if full-resolution, uncompressed data would generate larger than manageable file sizes at a larger tile size.
This index level is useful for users of LiDAR-derived elevation data stored as TINS because TIN format data, as well as ASCII format data, are stored only at this tiling level. Imagery in native uncompressed format will also be stored only at this level.
As each cell is defined with orthogonal edges relative the Washington State Plane coordinate system, it accomodates most common pixel resolutions of multiple ground sample distance (gsd) without partial pixel effects and without zero or NULL data 'splinters'.